Obama alums: We're Ready for Warren
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More than 300 former Obama campaign staffers have signed a letter urging Sen. Elizabeth WarrenElizabeth WarrenWarren calls for US to support ceasefire between Israel and Hamas Sanders: Netanyahu has cultivated 'racist nationalism' Tensions mount among Democrats over US-Israel policy MORE (D-Mass.) to run for president, part of a growing effort to get the liberal champion to run. 

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The letter, posted Friday on Ready for Warren's website, cites President Obama's come-from-behind victory over Hillary ClintonHillary Diane Rodham ClintonCongress won't end the wars, so states must Democrats say it's up to GOP to stop Trump 2024 Hillary Clinton to speak at Asian Americans and Pacific Islanders summit MORE in the 2008 primary, as some Democrats look for a challenger to Clinton once again. 

"We believed in an unlikely candidate who no one thought had a chance," the letter states. "We worked for him — and against all odds, we won in Iowa. We organized like no campaign had organized before — and won the Democratic primary."

"We know that the improbable is far from impossible," it adds. 

The signers include Rajeev Chopra, Obama's chief information officer in both campaigns; Stephen Geer, director of online fundraising in 2008; and Catherine Bracy, director of the tech field office in San Francisco in 2012. 

However, many of the more prominent Obama campaign names have lined up with the Clinton campaign-in-waiting. Jim Messina, Obama's 2012 campaign manager, signed on this year as co-chairman of the pro-Clinton super-PAC Priorities USA. John Podesta, currently an adviser to Obama, is expected to leave the White House soon ahead of possibly chairing a Clinton campaign. 

Ready for Hillary has signed on with 270 Strategies, the firm of top Obama campaign aides Mitch Stewart and Jeremy Bird.

The letter is far from the only effort urging Warren to run. Liberal groups, including MoveOn.org and Democracy for America, this week began campaigns to convince Warren to enter the race. 

Warren has repeatedly said she is not running and has given few indications that she is preparing to jump in.

There are still some on the left who argue Clinton is not strong enough on fighting income inequality and too close to Wall Street. 

"Rising income inequality is the challenge of our times," the former Obama staffers' letter states. "And we want someone who will stand up for working families and take on the Wall Street banks and special interests that took down our economy."